Walmart’s $45 refill cards, which used to provide 1 GB of usage, now offers users 4 GB of usage for the same money. Best of all, unlike other prepaid data plans, any unused data can be rolled over, and used in the next month.

Sprint’s iPad and iPad mini plans cost 300MB/$14.99, 3GB/$34.99, 6GB/$49.99, or 12GB for $79.99. There are also $10 and $15 offers for customers that also have Sprint smartphones that offer 100MB and 1GB of data, respectively, and activation fees for all 3G/4G tablets are being waived for a limited time.

Solavei launched in September with a $49 unlimited voice, text and data plan. The MVNO rides on T-Mobile USA’s GSM network with a $49 startup fee. The company sells HTC One S, HTC Wildfire or ZTE Origin smartphones, which range in price from $160 to $500.

Republic Wireless pledged “unlimited” voice and data service for $19 per month, but only if most traffic was routed to Wi-Fi. Currently, customers pay $199 up front for their first month of service and for the LG Optimus smartphone running version 2.3 Android. If customers want to continue with the no-contract service, they can pay $19 a month plus taxes after that, but can cancel the service at any time without an early termination fee.

FreedomPop. FreedomPop’s $99 iPhone case promises free broadband. You can get up to 500 MB free every month using their mobile WiFi unit that uses a built-in WiMax radio.

Ultra Mobile, a T-Mobile USA MVNO, offers a prepaid unlimited plan that includes up to 1,000 minutes of international calling. It operates on a bring-your-own-device, SIM-only model and has a $29 per month unlimited talk and text and $39 per month for unlimited talk, text and HSPA data.